An ultrasonic wave Doppler diagnosis instrument has become an indispensable modality at the time of observing a blood flow in a subject to be examined. In the ultrasonic wave Doppler diagnosis instrument, an X-ray exposure as in an X-ray diagnostic apparatus is not performed. A probe comes into contact with a body surface of the subject to be examined, so that the subject can be diagnosed. Therefore, it can meet a requirement for convenience in a medical field.
As an ultrasonic wave Doppler method which is performed by the ultrasonic wave Doppler diagnosis instrument, there are a pulsed wave Doppler (PWD) method and a continuous wave Doppler method (CWD). Of them, according to the pulsed wave Doppler method, using a pulsed wave as a transmission ultrasonic wave, only information of a blood flow of any specific depth can be detected. The pulsed wave Doppler method is mainly used in the heart or great arteries. However, in the pulsed wave Doppler method, since there is a restriction by a pulse repetition frequency (PRF), a so-called turnback phenomenon is generated. As a result, there is a restriction in a range of measurable blood flow speed.
In the meantime, according to the continuous wave Doppler method, a continuous wave is used as a transmitted ultrasonic wave. However, in processing a reflective ultrasonic wave signal where all Doppler signals in an ultrasonic beam direction overlap, there is a problem in that there is no distance resolution. In addition, since all reflective signals on a beam are collected, a clutter component of large power other than a Doppler component is contained. As a result, in obtaining sufficient Doppler sensitivity by using the continuous wave Doppler method, a large dynamic range or a steep wall filter is necessary. Therefore, the continuous wave Doppler method is mainly used for speed detection of a valve regurgitation jet of the heart or the like.
However, different from the pulsed wave Doppler method, the continuous wave Doppler method has a distinct advantage. That is, according to the advantage of the continuous wave Doppler method, the collection of blood flow information using a continuous wave can be made. Therefore, a Doppler range can be increased (specifically, a sampling frequency of a frequency analyzer is an upper limit), and a spectrum Doppler image can be obtained in which turnback does not occur even in a high-speed blood flow like a jet flow.
In the meantime, in a recent ultrasonic wave Doppler method, the needs have been required ‘that want to detect fast blood flow deep within a subject to be examined without turnback’. These needs cannot be satisfied by the above-mentioned pulsed wave Doppler method and continuous wave Doppler method. Accordingly, some attempts have been made so as to meet these needs. One of some attempts is an HPRF (High PRF) method of a pulsed wave Doppler. However, even though the HPRF method is used, it is likely for a second range gate to cause saturation in a short distance, and signals are not necessarily accurately collected from a deep portion of the subject.
In addition, another method for meeting the above-mentioned needs has been disclosed in Patent Documents 1 and 2. This method is based on a focus technology by a continuous wave Doppler method using an ultrasonic wave probe of a two-dimensional array. Specifically, according to this method, focus locations of transmission and reception are varied by means of not a conventional one-dimensional ultrasonic wave probe but a 1.5 dimensional or two-dimensional array-type ultrasonic wave probe having an array expanded in a lens direction (elevation direction), and a Doppler signal near the focus is selectively collected.
However, in the focus method disclosed in Patent Documents, the sensitivity near the focus is raised slightly, as compared with the sensitivity of a range other than the range near the focus. When a fixed material component/moving material component having large power exist on the same beam, it is not possible to obtain a sufficient distance resolution. In addition, since the detection is made with the sensitivity distribution that is determined by a sound field distribution of transmission and reception near the focus, a signal of a region having a large width may be detected, which results in not obtaining the sufficient distance resolution.    Patent Document 1: U.S. Pat. No. 2,500,937    Patent Document 2: U.S. Pat. No. 3,069,910